A superabsorbent polymer (SAP) is a synthetic polymeric material capable of absorbing moisture from about 500 to 1000 times its own weight. Various manufacturers have denominated it as different names, such as SAM (Super Absorbency Material), AGM (Absorbent Gel Material), etc. Since such superabsorbent polymers started to be practically applied in sanitary products, now they have been widely used not only for hygiene products such as disposable diapers for children, sanitary napkins, etc., but also for water retaining soil products for gardening, water stop materials for the civil engineering and construction, sheets for raising seedling, fresh-keeping agents for food distribution fields, materials for poultice, etc.
In most cases, these superabsorbent polymers have been widely used in the field of hygienic materials such as diapers, sanitary napkins, etc. For these applications, the superabsorbent polymers are required to exhibit a high absorption rate with respect to moisture, etc., and also required to exhibit an absorption rate above a predetermined level even under an external pressure or in a partially swollen state.
Therefore, in order to improve the absorption rate of superabsorbent polymers, studies have been continued on a technology of increasing the absorption surface area of superabsorbent polymers.
As a method of improving the absorption rate by increasing the absorption surface area of superabsorbent polymers, a method of forming many pores inside the superabsorbent polymer to rapidly absorb water or a method of preparing the superabsorbent polymer as small particles to improve a contact surface area with water may be considered.
As the former method, a method of preparing a superabsorbent polymer by using a foaming agent, etc. was suggested, but bubbles generated by the foaming agent were not sufficiently included inside the superabsorbent polymer, and it was very difficult to control a size of the pore formed inside the superabsorbent polymer by the known method.
Further, the superabsorbent polymer prepared by the known method had a slightly improved absorption rate due to a porous structure, but the porous structure formed by the known method had weak gel strength, and thus there were problems that absorbency under load and liquid permeability were deteriorated. That is, when the former method was used, a desired level of absorption rate could not be achieved, and it was difficult to provide a superabsorbent polymer having predetermined levels of physical properties.
Meanwhile, since there is a technical limitation in controlling the superabsorbent polymer to have a small particle size, the latter method may not sufficiently increase the absorption surface area of the superabsorbent polymer. Accordingly, there is a demand for studies to increase the absorption surface area of the superabsorbent polymer while maintaining physical properties thereof.